1. Field of the Invention
The present invention relates generally to fuel assemblies for nuclear reactors and, more particularly, is directed to a nuclear fuel rod having an improved end plug assembly which provides a cavity for additional fission gas space, reduces stress concentration in the fuel rod end and establishes a barrier to fuel chip entry into the cavity.
2. Description of the Prior Art
As well known in the art, a fuel rod for use in fuel assemblies of a nuclear reactor includes a plurality of cylindrical nuclear fuel pellets, such as composed of UO.sub.2 enriched with U-235, disposed end to end within a tubular cladding member or fuel tube. The fuel tube is an elongated thin-walled tube, preferably of a zirconium alloy. The opposite ends of the fuel tube are closed by upper and lower end plugs, preferably formed of the same material as the fuel tube.
It is well known that the overall efficiency of a nuclear reactor can be increased and the useful life of its fuel rods prolonged if the fuel rods are internally pressurized. Thus, during fabrication of a fuel rod an inert gas, such as helium, is introduced into the fuel tube under pressure after which the end plugs are welded in the tube ends to seal the tube. Additional internal pressure within the tube arises during operation of the nuclear reactor wherein gasses are generated over the life of the fuel rod. Toward the end of the fuel rod's life, its internal pressure can reach as high as 1000 psi.
During operation of the reactor the higher pressure of the coolant (approximately 2500 psi) on the exterior of the fuel rods normally offsets the internal pressure of the fuel rods. However, during shut down of the reactor, the external pressure of the coolant decreases to zero. Then, the internal pressure of the fuel rod causes outward expansion of the tube. In one prior art fuel rod having a solid end plug inserted into the fuel tube end and welded thereto at the juncture of the tube end and the plug periphery, outward expansion of the tube results in the location of a stress riser and the concentration of the point of maximum discontinuity stress at the weld joint. As long as the tube material and stresses are within the fatigue life limits, the weld joint will not fail. However, if the weld is imperfect or the material loses ductility due to hydriding and/or irradiation hardening, the weld joint may fail.
One solution is to form a cavity within the end plug to provide extra space for fission gas buildup and to form the weld joint between the fuel tube end and a thin section machined into the end plug. An end plug designed along these lines is disclosed and illustrated in U.S. Pat. No. 3,679,545 to Leirvik. This design embodies advantages from a stress standpoint as well as a welding standpoint. From the stress standpoint, additional space provided by the cavity in the end plug reduces the pressure of gas buildup and thereby relieves expansion of the fuel tube, to some degree, during reactor shut down. Also, although this design has the same discontinuity stress, the stress riser due to the end plug-to-fuel tube junction does not occur at the point of maximum discontinuity stress, since the latter is located farther outwardly along the end plug within the cavity. Therefore, the maximum discontinuity stress is not increased by a stress concentration factor as was the case in the solid end plug design. From the welding standpoint, parts of approximately equal thicknesses are easier to weld together than parts which have a large difference in their respective thicknesses.
However, some of the advantages of the design are offset by the replacement of some of the fuel pellets by ceramic pellets to thermally insulate the end plugs from the fuel pellets. This takes up valuable fuel pellet space in the fuel tube. Also, the design fails to make provision to prevent fuel pellet chips from falling into the end plug cavity at the lower end of the vertically-positioned fuel rod. Chips in the end plug cavity causes an additional problem of how to remove the heat generated by the chips in the end plug.
Consequently, a need exists to improve the design of end plugs used in fuel rods so as to add space to accommodate fission gas buildup without increasing fuel rod length or removing fuel pellets. In such improved design it is desirable to retain and enhance the positive features of prior art plug designs without at the same time assuming the burdens of their negative features.